1. Field of the Invention:
The invention relates to the solvent-free manufacture of aqueous, radiation-curable urethane acrylate emulsions of products from the reaction of a cyclic polyisocyanate with water, an hydroxyethyl acrylate, and a polyetherpolyol, in the presence of an aqueous nonionic surfactant.
2. Discussion of the Background:
Urethane acrylates are often used in forming coatings, which are radiation curable. However, their viscosity is often such that urethane acrylates cannot be directly used in doctoring, pour molding, or injection molding, except with the use of low-viscosity thinners. Classical organic solvents are excellent thinners, but prior to radiation-curing they must be evaporated, which is a disadvantageous process. Low molecular weight olefins are also excellent thinners or viscosity regulators, due in part to their solvating capability. But they tend to have high boiling points so that they do not evaporate prior to curing, but interact with the components of the dissolved urethane acrylate during radiation curing to yield polymers. Lower molecular weight olefinic thinners, which do have lower vapor presence, are toxic, give off an unpleasant odor, and are environmentally unacceptable. These problems, along with the trend toward conservation of raw materials and energy, have led to the development of aqueous emulsions which are radiation-curable.
The emulsification of acrylic resins in water can be accomplished with acid resin systems and subsequent neutralization with bases to form anionic emulsions, or with basic resins and subsequent neutralization with acids to form cationic emulsions. Aqueous, ionic solvent-free systems have a number of advantages, as well as certain disadvantages compared with coatings which are applied from solvent-containing systems.
Water-based coatings are increasingly desired, as an alternative to organic solvents for ecological reasons, but solvent-containing coatings still possess superior coating properties, often better than those in aqueous systems.
Commonly known disadvantages of many aqueous systems are their tendency to absorb water. Coatings formed from aqueous systems are known to possess poor weather-resistance, generally attributed to the ionic components of such systems. These problems substantially reduce the areas of potential applicability.
It was considered therefor to use nonionic surfactants to convert the highly viscous resin systems of the prepolymer into stable emulsions with water and thereby eliminate or substantially eliminate the organic solvent.
It has been proposed to emulsify such resins in water without the use of organic solvents, if one uses, e.g., alkyl phenoxylates or polyether polyols. However, the shelf stability of such preparations is limited to several hours or 1-2 days, and hence, they must be used rapidly after formulation.
Improved shelf stability is possible with the aid of, for example a commercially available polyurethane thickener and while such systems provide transparent films of good solvent resistance after physical drying and radiation-curing, their adhesion to smooth substrates is generally poor. Surprisingly, if one tests the water absorptivity of such films, e.g. in 100% relative humidity air, one finds that it has a "blockwise pattern" to the same degree as often occurs in the case of ionic, aqueous systems, where the surface is softened; which means that the coatings are no longer "stackable". Also, they cause pollution.
If one uses a certain proportion of a water-soluble aliphatic alcohol, one can substantially improve the adhesion of the nonionic aqueous emulsion. However, the flash point of such preparations will be undesirably lowered, and residual amounts of alcohol will remain in the cured coating which can promote water absorption by the films.
Thus, it is very difficult to identify a suitable nonionic surfactant which
(1) is compatible with urethane acrylates,
(2) provides stable aqueous emulsions which yield, after curing, a final film which adheres to the substrate and is elastic, and
(3) does not promote water absorption by the coating.
Also, the high melt viscosity of most known aqueous solvent-free emulsions of prepolymeric urethane acrylates or urethane urea acrylates requires that the emulsification be carried out in the temperature range 100.degree.-120.degree. C., i.e., under pressure. In view of the risk of thermal polymerization of the acrylic compounds one cannot exploit these products.
While low molecular weight urethane acrylates can be more readily processed, they yield hard, nonadhering films due to the nature of the resin. It is very difficult to achieve thinning of urethane acrylates using organic thinners, e.g., by partial urea formation with the use of diamines, because the reaction is so vigorous, that the viscosity increases to the point that the material cannot be stirred, and the prepolymer forms a gel as a result of heat localization.
Accordingly, one problem addressed by the present invention is the identification of suitable olefinic nonionic surfactants, which copolymerize with suitable urethane acrylate resins to prevent reemulsifiability, water absorption, and "sweating" of the surfactant out of the coating.
This problem is solved by the use of a hexose alkoxylate ester of an unsaturated fatty acid, as a nonionic surfactant, in solvent-free aqueous urethane acrylate emulsions.
Surprisingly it was found that, beginning with a cyclic diisocyanate or a partially hydrolyzed diisocyanate (to form an urea/diisocyanate), followed by acrylation with hydroxyethyl acrylate, and reacting with a polyol, one can produce a meltable compatible urethane urea diacrylate which can be converted into an aqueous emulsion using the above-mentioned olefinic nonionic surfactant. If avoidance of urea formation is desired, the urethane acrylates produced with the surfactants employed in the present invention, in the absence of water, also have acceptable properties. On the other hand, if urea-containing urethane acrylates are used (see Examples 1 and 2), there are substantial improvements in ultimate tensile strength, elongation at failure, Erichsen cupping, and hardness (pendulum test).